It is often desirable to produce black-and-white photographic images formed by a combination of maximum density areas and minimum density areas e.g. half tone imaging. For such imaging applications a contrast of at least 10 (herein referred to as high contrast) and more typically near or above 20 is employed. An example of high contrast photographic elements having white reflective supports are phototypesetting materials intended to produce black type character images on the white background. An example of high contrast photographic elements having transparent supports are lith films, so called because they are used as contact transparencies for exposing lithographic printing plates. The illusion that some areas of a printed image are of intermediate density is created by the viewer's inability to resolve tiny dots of maximum density and background areas of minimum density that separate them.
The use of hydrazines in the developer and/or photographic elements of high contrast systems to increase speed and contrast is well known and disclosed for example, in British Patent No. 598108, U.S. Pat. Nos. 2322027, 2419974, 2419975, 4166742, 4168977, 4211857, 4224401, 4243739, 4272606, 4272614, 4311781 and 4323643 and in Research Disclosure, Vol. 235. November 1983, Item 23510.
In surface latent image forming silver halide emulsions, the grains which are exposed to light are rendered developable while grains which are not exposed to light are not intended to be developed. Nevertheless some of these unexposed grains develop spontaneously. In full tone imaging the spontaneously developing grains raise minimum density more or less uniformly. Such minimum density levels are referred to as fog and, so long as they remain low are not objectionable.
Pepper fog differs from ordinary fog in that it takes the form of small, maximum density areas randomly distributed on a substantially uniform minimum density background. When a photographic element exhibiting pepper fog is viewed under magnification the impression to the viewer is often that the magnified field of view has been sprinkled with grains of pepper.
Pepper fog is a well recognised problem in high contrast photographic systems and provides a serious problem to the photographic printing plate making process. These black spots are tiny black specks which appear in the area between dots that is not intended to be developed. The tendency to form spots increases and grows on ageing of the photographic material particularly during storage thereof under high temperature and/or high humidity conditions, or as the concentration of the sulfite ion used commonly as a preservative in the developer decreases or as the pH value of the solution increases. The formation of black peppers detracts considerably from the marketability of the product as a photographic material for manufacturing a photographic printing plate. Many efforts have been made to overcome this black pepper problem but improvement in black pepper is often accompanied by decreases in sensitivity and gamma (contrast), and there has been a strong demand for a photographic system with reduced black pepper which does not entail losses of sensitivity and high contrast.
U.S. Pat. Specification No. 4618574 discloses a negative working photographic element capable of producing a high contrast silver image. The element comprises surface latent image forming monodispersed silver halide grains having a mean diameter of less than 0.7 microns a contrast enhancing arylhydrazide, and in an amount sufficient to reduce pepper fog while maintaining high contrast, a polyhydroxybenzene and a carboxyalkyl-3H-thiazoline-2-thione.
European Patent Application, Publication No. 0196626 discloses a silver halide photographic material comprising a support, at least one silver halide emulsion layer and one or more light-insensitive hydrophilic colloid layers, wherein said silver halide emulsion layer or said light-insensitive hydrophilic colloid layer contains a hydrazine derivative, and the photographic material has a film surface pH not higher than 5.8 on the side of said emulsion layer inclusive of said light-insensitive hydrophilic colloid layer. The formation of pepper fog is reduced by maintaining the pH of the film surface on the side of the emulsion layer at a level not more than 5.8.
It is an aspect of the present invention to provide new high contrast photographic materials having a reduced propensity to formation of pepper fog.